1. Field of the Invention
This invention relates generally to a method for the recovery of gold from refractory ores. More specifically, this invention relates to a method for the recovery of gold from refractory sulfidic ores which contain large quantities of metal carbonates such as dolomite.
2. Prior Art
In recent years the mining industry has become increasingly involved with developing methods for extracting gold from complex refractory ores. This trend is due to the increasing scarcity of free milling gold ore deposits. Gold ores are refractory when their gold content is closely associated with metallic sulfides in the ore. The metallic sulfides found in gold ore are substantially pyrite and pyrite derivative compounds such as arsenopyrite. Although sulfidic refractory ores have been known and studied for years, the actual chemistry involved with their refractory nature is not clearly understood. Additionally, sulfidic ores vary considerably between deposits in both the refractory nature and the mineral content of the ore. The mining industry has developed a number of different processes to extract gold from sulfidic ores.
U.S. Pat. No. 4,266,972 to Redondo-Abad et al. discloses a method for treating a sulfidic ore containing precious metals and base metals under an oxygen pressure of about 5 to 15 Kg/cm.sup.2 g (71 to 213 psig) at a temperature of 150.degree. C. to 250.degree. C. The sulfidic ore is leached under these conditions by water to dissolve zinc and copper, hydrolyze iron to hematite, and leave lead and precious metals as insoluble sulfates. The lead, silver, and gold are leached from a solid residue by a strong chloride solution containing calcium chloride at a temperature in the range of about 60.degree. C. to 90.degree. C., at a pH in the range of about 7.0 to 1.0 and in the presence of about 1 gram per liter of ferric chloride.
U.S. Pat. No. 4,431,614 to Makipirtti et al. discloses a method for the separation of gold and silver from complex sulfide ore. The method of this invention involves heating the sulfide ore at a temperature of 600.degree. C. to 900.degree. C. at a sulfur pressure of 0.2 to 1 atmosphere to bring the complex metal compounds to a suitable form for a subsequent alkaline cyanide leaching. The gold and silver is then dissolved by a cyanide solution and separated from the insoluble residue. This method uses a leaching time of about 8 hours and obtains yields of silver at about 48 percent and yields of gold at about 96 percent.
U.S. Pat. No. 4,438,076 to Pietsch et al. discloses a method for extracting gold and silver from an ore. This method involves a process for leaching gold and silver from an ore in an alkaline cyanide solution. The leaching is performed by maintaining the slurry in a turbulent state at a pressure of 25 to 130 bar (363 to 1886 psig) while injecting oxygen into the slurry as the slurry passes through a tube reactor in a continuous unidirectional flow. The oxygen having a purity of at least 90 percent is injected such that the CN/O.sub.2 molar ratio is at most 0.7 at 25 and 0.7 g/l NaCN. The CN/O.sub.2 molar ratio is lower than 0.7 at pressures higher than 25 bar. The slurry temperature is 70.degree. C. or less during this reaction. Lime can be used to adjust the slurry pH. The method requires a tube reactor which does not readily lend the process to large scale industrial gold extraction operations.
U.S. Pat. No. 4,442,072 to Baglin et al. discloses a method for the selective recovery of base metals and precious metals from ores. The ores are smelted with a flux to form a matte and slag. The matte is subsequently ground and leached with about 10 to 40 weight percent sulfuric acid at a temperature of about 40.degree. C. to about 100.degree. C. at atmospheric pressure. This step selectively solubilizes nickel and iron in the ore after leaching. The ore is roasted at about 300.degree. C. to 500.degree. C. and further leached with dilute sulfuric acid at ambient temperature and pressure to extract copper. The remaining residue has a high concentration of platinum, palladium, and gold.
Extracting gold from refractory sulfidic ores is made more difficult when the ores contain large amounts of basic metal carbonates. One of the most common metal carbonates found in gold ores is dolomite. Pressure leaching processes using acid as the leaching agent have been used with refractory sulfidic ores. An acid pretreatment step can be utilized when there is only a small amount of dolomite in the ore.
U.S. Pat. No. 4,084,961 to Caldon discloses a method for the treatment of metal bearing mineral material. This invention is a pressure leaching process that can be preceded by an optional acid pretreatment step wherein sulfuric and nitric acids are used to remove unwanted metal carbonates from the ore. This reaction is conducted under pressurized oxygen at elevated temperatures. Ores having high concentrations of carbonates consume a correspondingly higher amount of acid in pretreatment steps. This adds to the overall expense of the gold leaching operation. In addition, the use of corrosive acids in either a pretreatment step or in a pressure leaching step can necessitate the use of expensive corrosion resistant materials in the equipment in which the acid treatment operation is conducted.
An example of an economical method for the recovery of gold from refractory carbonaceous ores is U.S. Pat. No. 4,289,532 to Matson et al. herein incorporated by reference. Refractory carbonaceous ores owe their refractory nature to their carbon content rather than to their sulfidic content as do the ores whose recovery is the object of the present invention. This method subjects ores to simultaneous cyanidation and countercurrent granular activated carbon adsorption in two or more stages. Prior to cyanidation this method includes an oxidation procedure, such as an oxygenation and/or chlorination procedures to make the carbonaceous ore more amenable to cyanidation. This method has been proven to be an economical process for recovering gold from very refractory carbonaceous ores, but provides less than desirable results with refractory ores that possess both sulfidic and carbonaceous characteristics.
The industry is lacking an effective, acid-free method of recovering gold from sulfidic ores which are refractory to conventional cyanidation techniques and which contain large quantities of basic metal carbonates.